1. Field of the Invention
The present invention relates to a sheet alignment apparatus and a sheet post-processing apparatus provided with the sheet alignment apparatus.
2. Description of the Related Art
In a sheet post-processing apparatus that performs so-called post-processing such as stapling or punching for a sheet bundle composed of collected sheets on which images are formed, a sheet alignment apparatus is used to align the edge of the sheet bundle at a predetermined alignment position before the post-processing.
FIG. 11 is a vertical cross-sectional view showing a configuration around a sheet alignment apparatus in a conventional sheet post-processing apparatus.
In the conventional sheet alignment apparatus shown in FIG. 11, a sheet S to be aligned is passed through a predetermined feeding path and guided to a stacking tray 4 by a feeding roller 303. As shown in FIG. 11, the stacking surface of the stacking tray 4 is sloped down to a stopper portion 21 and, accordingly, the sheet S on the stacking surface slips down by its own weight to the stopper portion 21.
A knurled belt 302 and a sheet discharge roller E feed the sheet S thus slipping down to the stopper portion 21 to allow it to be butted against a stopper 211 of the stopper portion 21. Thus, the downstream edge of the sheet in the feeding direction thereof stops at a predetermined alignment position (butting surface of the stopper 211).
The sheets are thus sequentially butted against the stopper 211 and sequentially stacked on the stacking tray 4. Meanwhile, each of the sheets onto which a toner image has been heat-fixed may be curled in some case. Therefore, in order to correctly align the edge portion of the sheet bundle at a predetermined alignment position even in the case where each of the sheets is curled, a press portion 1J made of an elastic plate member is provided to press the downstream edge of the sheet bundle whose end portion has been allowed to be butted against the stopper 211 downward against the tray 4. On the other hand, in order to align the sheet position in the direction perpendicular to the feeding direction R of the sheet S, a not-shown alignment plate capable of shifting in the direction perpendicular to the feeding direction is provided to press the edge potion of the sheet S in the direction perpendicular to the paper surface in FIG. 11 to a not-shown reference plate.
In this sheet alignment apparatus, the above sheet alignment operation is performed for each sheet until the number of sheets to be stacked on the stacking tray 4 has reached a preset number of sheets (the unit of sheets to be post-processed). The sheet bundle that has been aligned in the sheet alignment apparatus is subjected to predetermined post-processing in the sheet post-processing apparatus and then discharged on a discharge tray 5 by the discharge roller E and a feeding roller 301a of a swing guide 301 provided in the sheet post-processing apparatus.
The number of sheets constituting a sheet bundle to be aligned in the sheet alignment apparatus varies depending on circumstances. It is possible to align (eliminate the curl) the downstream edge of the sheet with a comparatively low pressing force in the case where a small number of sheets are to be aligned; whereas a higher pressing force is required in the case where a small number of the sheets is to be aligned.
Setting for the press portion 1J can be made such that a high pressing force is always applied to a sheet bundle so as to cope with a large number of sheets. However, if so, the pressing force may become overload in the alignment operation of the alignment plate, preventing stable alignment operation (particularly, in the case where the number of sheets to be aligned is small, sheets may be bended in the feeding direction by the alignment plate, and sheet may be bounded when the pressing force of the alignment plate is released to cause malfunction in the alignment operation).
As a method that can solve the problem relating to a relationship between the number of the sheets to be aligned and pressing force of the press portion, the following prior arts have been known: one that changes the pressing force of the press portion according to the number (height) of sheets constituting a sheet bundle (Jpn. Pat. Appln. Laid-Open Publication No. 11-130338), one that changes the pressing force for holding a sheet bundle, which is given by a feeding roller that feeds a sheet to a predetermined alignment position, according to the stacking height of the sheet bundle (Jpn. Pat. Appln. Laid-Open Publication No. 2000-351522), and one that counts the number of sheets that have been stacked on the stacking tray and brings the sheet bundle close to a predetermined position (clinching surface) at the time point when a predetermined number of sheets are stacked so that a sheet bundle having a large thickness can be stapled without fail (Jpn. Pat. Appln. Laid-Open Publication No. 9-216764).
However, in the above configurations, it is necessary to add a special component such as a means for detecting the height of a sheet bundle or number of the staked sheets or a mechanism for changing the pressing force given by the press portion, which may prevent miniaturization and cost reduction.